fix(interpreter): bound recursive prompt expansion

[chaliy]

Summary

Bounds recursive prompt expansion so the reported self-referential payload can no longer recurse without limit. Root cause: `` re-enters prompt decoding on every cycle (expand_word -> prompt_transformation -> expand_prompt_string -> expand_word) with no depth tracking, so a self-referential value recurses until the native stack overflows and aborts the host process.

Reported vector (default promptvars on):

x='${x@P}'; echo "${x@P}"

Changes

• Depth guard on the shared prompt-decode path: nesting deeper than 8 fails closed with a bounded message instead of recursing; depth unwinds on exit.
• Recursion budget pre-charged before UTF-8 decode; decoded input charged so hostile overlong prefixes cannot hide from the cap; child re-parse shares the active budget/depth and fails closed on a poisoned budget.
• Cancellation honored through prompt evaluation.
• ExecutionMode::for_prompt(): prompt path can never enable nested execution.
• Limits docs/callouts, spec case, knowledge updates.

Before / After

• Before (per report): the payload above recurses without bound until the native stack overflows and the process aborts.
• After (verified at merge commit): the same payload exits 1 in ~10ms with a 118-byte actionable diagnostic (resource limit exceeded: maximum prompt expansion depth exceeded (8)); stdout empty; subsequent invocations run normally. 7/7 new prompt_resource_tests pass (self/mutual cycles, finite chains, depth boundary ok/rejected, sibling independence, session recovery, shared budget), plus 6 prompt unit, 81 tool/limits, 203 parser tests; cargo fmt --check and cargo clippy -p bashkit --all-targets clean.

Tests

cargo test -p bashkit --test integration prompt_resource (7 passed)
cargo test -p bashkit --lib prompt (6 passed), limits (81 passed), parser (203 passed)

Produced by yolop

[cloudflare-workers-and-pages]

Deploying with    Cloudflare Workers

The latest updates on your project. Learn more about integrating Git with Workers.

Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	839c036	Commit Preview URL Branch Preview URL	Oct 10 2026, 11:40 PM